Optical reflectivity and electronic structure of layered cadmium halides

The dielectric function {ε}^(E), the energy loss function -Im[1/ {ε}^(E)], and plots of neff(E) and εeff(E) have been obtained in the region 2--31 eV by Kramers-Kronig analysis of near-normal-incidence reflectance spectra of single crystals of CdCl₂, CdBr₂, and CdI₂. The results have been described in terms of interband transitions and plasma oscillations. The low-temperature spectra of the materials reveal the presence of strong exciton structures. Plasma resonance effects have been identified in the high-energy region (15--22 eV). The optical spectra of CdX₂ (X=Cl, Br, I) were analyzed in terms of the calculated band structures of CdCl₂ and CdI₂. In CdI₂ the smallest energy gap for forbidden (phonon-assisted) direct transitions (at the L point of the Brillouin zone) is 3.8 eV (300 K), while the gap for allowed direct transitions (at {Γ}) is 4.3 eV (30 K). The fundamental energy gap in CdCl₂ and CdBr₂ is observed at 6.4 and 5.4 eV, respectively, and corresponds to allowed direct transitions at {Γ}. The observed excitons X₀, X₁, and X₂ are associated with allowed direct transitions between the halogen np valence band and the Cd 5s conduction band. The excitons A, B, and C are due to transitions of electrons from the halogen valence band to the Cd 5p conduction band.